Add appdata file for the Fedora Software center

Solves #960

@Ablu Would such an appdata file be sufficient?

Anyway we should have our own quality screenshots in our wiki, not linking to tiled.
Correct. I think both repos can have an appdata file like this, but only flare-game needs to specify a desktop file.

By the way, I think we should use a different screenshot, since that OSARE one is obviously out of date.